Method and solutions for treating titanium and like metals and their alloys

ABSTRACT

PARTS FORMED OF TITANIUM OR OTHER GROUP IV TRANSITIONAL SUB-GROUP METALS AND THEIR ALLOYS ARE PREPARED FOR ELECTROPLATING, ANODIZING, PAINTING, ADHESIVE BONDING, AND OTHER SURFACE PROCESSING BY IMMERSING SAID METALS IN A HOT AQUEOUS SOLUTION CONTAINING A HYDROXIDE OF AN ALKALI METAL, A CHELATING AGENT, AND AT LEAST ONE OF THE THREE TRIHYDROXYBENZENES OR ONE OF THE METHYL SUBSTITUTED ALPHA OR BETA NAPHTHOLS.

United States Patent Ofice 3,687,741 Patented Aug. 29, 1972 3,687,741 METHOD AND SOLUTIONS FOR TREATING TITANIUM AND LIKE METALS AND THEIR ALLOYS Earl W. Kendall, Bonita, Calif., assignor to Rohr Corporation, Chula Vista, Calif. N Drawing. Filed Sept. 22, 1969, Ser. No. 860,069 Int. Cl. C23f 7/02 US. Cl. 1486.14 R 20 Claims ABSTRACT OF THE DISCLOSURE This invention relates to surface processing of parts formed of titanium, zirconium, hafnium, thorium and alloys thereof, and more particularly to a method and solutions for forming on these metals an oxide film which permits electroplating, adhesive, paint, and other coatings to be effectively bonded thereto.

Many surface finishing and bonding techniques which are widely used in the manufacture of steel and aluminum parts cannot easily be used in connection with metals in the Group IV transitional sub-group of the Periodic Table, namely, titanium, zirconium, hafnium, and thorium. This ditficulty results from the formation of oxide films on the latter metals which provide unsatisfactory surfaces for electrodeposition of metals, or for adherence of paints, anodic films, dry film lubricants, adhesives and the like. It has been the practice in accordance with the prior art to perform two separate processing steps in preparing titanium (which is being increasingly substituted for aluminum in the fabrication of many aircraft components) for the attachment thereto of coatings of the aforenamed types. First the titanium part is cleaned by applying an organic solvent or an alkaline cleaner to its surface, and then the part is generally immersed in a bath containing hydrofluoric and nitric acids in various ratios, the fluoride ion being evolved from salts such as ammonium bifluoride and the sodium, potassium, or ammonium fluorides, or from fluoboric acid. The described treatment satisfactorily removes oxides from the surfaces of titanium parts. However, numerous investigators have shown that re-oxidation of titanium occurs rapidly after it is removed from a reducing bath, and thus subsequent coating or bonding processes cannot be successfully accomplished unless the cleaned and deoxidized titanium is quickly transferred to another bath while it is wet with the reducing composition, or elaborate precautions are taken to prevent air from contacting the tianium while it is stored for subsequent processing. Furthermore, it has been found that the chlorinated hydrocarbons generally used in vapor degreasing of metallic parts break down during such cleaning operations to form small amounts of hydrochloric acid, which acid adversely affects titanium in that it causes incipient stress corrosion cracking. Other organic solvents used for cleaning metals are rather expensive and frequently selective as to the types of contaminants which they will remove. In addition, the hot and cold alkaline compounds used to clean titanium are incapable of completely removing certain surface contaminants such as oils and greases.

In recent years conversion films have been applied to titanium to produce thereon a surface to which adhesives can effectively be bonded, the conversion films retarding the rate of re-oxidation of the titanium when it is exposed to air. The well-known PASA-JELL (trademark of Semco Division of Products Research and Chemical Corporation) and phosphate-fluoride treatments for applying conversion films to titanium are disadvantageous in that parts must be bonded within 12 to 15 hours after the films are applied, otherwise the titanium must be completely reprocessed. There are other methods for applying conversion films to titanium which utilize citric and oxalic acids. However, prior to the use of these acids, the surface of the workpiece must be roughened, which is generally accomplished by sand or dust blasting. Some of the particles used in this procedure remain on the surface of the titanium and subsequent exposure of the same to air or salt spray creates a discoloration thereon. In certain uses of titanium, such as in aircraft construction, this discoloration is unacceptable.

All of the above-mentioned disadvantages of previously known methods of preparing Group IV-A metals and their alloys for surface coatings are eliminated by the in vention disclosed herein, which only requires that such metals be contacted for a relatively short period with an alkaline solution after they have been cleaned. More particularly, this invention provides an alkaline solution that forms an oxide film on a Group IV-A metal when placed in contact therewith, and this film, unlike that which is formed on the metal when it is exposed to air after being deoxidized, provides a receptive surface for the many different finishes, coatings and adhesives commonly used in the manufacture of metallic products. To produce satisfactory results, parts which are treated by a solution in accordance with the invention must first be cleaned to remove all dirt, oil, grease and other contaminants from the surface thereof. Preferably, cleaning is accomplished by successively immersing the metal for about 1 to 5 minutes in the cleaner disclosed in US. Pat. No. 3,379,645 and for the same period in a modified version of the composition disclosed in US. Pat. No. 3,197,341. The metal is then immersed in an aqueous solution containing a hydroxide of an alkali metal, a chelating agent, and at least one of the three isomeric forms of trihydroxybenzene or one of the methyl substituted alpha or beta naphthols. If an adhesive, paint, dry film lubricant or other non-metallic coating is to be adhered to the treated metal, the latter is rinsed with demineralized water after it is removed from the aforesaid solution and then dried. If the part is to be anodized or electroplated, it is rinsed with demineralized Water after being removed from the solution and then immediately placed in the electroplating or anodizing bath without being dried.

The cleaning solution disclosed in the aforementioned US. Pat. No. 3,379,645 consists of from 15 to 25 parts by weight of chromic acid and from to 225 parts by weight of sulfuric acid. This chromic acid-sulfuric acid mixture is described in said patent in connection with the removal of paint films from different metals including titanium, but it has been found effective to use the mixture as a general cleaner for surfaces of titanium and the other metals included in the same sub-group of the Periodic Table. The acid bath may be at ambient temperature, and as previously stated, immersion of a workpiece therein for a period of about 1 to 5 minutes has been found sufficient for the purpose of the present invention.

After being immersed in the described acid bath, the workpiece of titanium, zirconium, hafnium or thorium is placed in a second acid bath consisting of ferric chloride, sulfuric acid, nitric acid, and water in the proportions specified in the aforementioned U.S. Pat. No. 3,197,341 but modified by the addition of ammonium bifluoride. It should be noted that the last-mentioned patent refers to this aqueous solution of ferric chloride and sulfuirc and nitric acids as a descaling solution for stainless steels and related alloys, but it has also been found effective to use the composition with modifications as a second bath preceding the process step of the present invention, which of course is directed toward the treatment of Group IV-A metals. The second bath may also be used at ambient temperature. Reference can be made to the specified US. patents for further details concerning the two acid baths which have been briefly described herein.

After being cleaned and deoxidized, a workpiece is immediately immersed in a bath having the following composition:

Component: Weight percent Alkali metal hydroxide 1.0-35.0 Chelating agent 1.0-4.0 Trihydroxybenzene 1.0-9.0 Water 52.0-97.0

Because of cost considerations, sodium hydroxide is the preferred alkali metal hydroxide for use in the processing solution of this invention. However, other alkali metal hydroxides, such as potassium hydroxide, may also be employed. The preferred chelating agent is sodium glucomate, but hydroxy acetic acid, the alkali salts of ethylene diamine tetraacetic acid, diethylenetriamine pentaacetic acid, and hexadecyltrimethylammonium bromide, or any other soluble material which serves as a chelating agent may be substituted. Without such a material in the solution the alkali metal hydroxide in the latter forms the hydroxide of the Group IV-A metal being treated, and this hydroxide appears as an insoluble material in the solution and its concentration increases as more metal is processed. However, the chelating agent attaches to the metal ion to form a soluble compound, and there is thus no adverse effect on the use of the solution. The trihydroxybenzene component of the processing composition may be 1, 2, 3 trihydroxybenzene (commonly called pyrogallol), l, 3, trihydroxybenzene (commonly called phloroglucinol), or the asymmetric form of the molecule, 1, 2, 4 trihydroxybenzene (commonly known as hydroxyhydroquinone). Mixtures of the three trihydroxybenzenes may also be used. It has also been found that the use in the compositions of methyl substituted forms of the naphthols in place of a trihydroxybenzene provides an equivalent surface preparation. The ranges of component concentrations which are given in the foregoing table provide a solution having a pH of approximately 12 /2 to 13. The order in which the named components are dissolved in water is immaterial. For best results, the temperature of the alkaline solution should be in the range of 175 to 200 F., and parts should be immersed from 5 to 15 minutes. As has been stated hereinbefore, parts to be coated with a non-metallic coating are water-rinsed and then dried after their removal from aforedescribed processing solution, whereas parts to be electroplated or anodized are also water-rinsed but are placed in their electrolytic processing baths without being dried.

The disclosed alkaline solution appears to form on titanium and other Group IV-A metatls an oxide film which is diiferent from that formed by exposure of said metals to air, and which presents no problem with respect to the electrodeposition of metals, adhesive bonding, adhesion of paint, dry film lubricants and the like, and the application of color by anodizing and other processes. Shear tensile strengths of adhesive bonds formed between specimens of titanium treated in accordance with this invention have been found to be 20 to 30 percent higher than the shear tensile strengths of adhesive bonds between identical specimens of the same metal which have been treated by the PASA-JELL or phosphate-fluoride 4 methods. The adhesion of electroplating and paint to titanium treated by the method of the invention has also been shown to be exceptionally good.

It is believed that the oxide film which is formed on the surface of a Group IV-A metal by the disclosed process includes the metal in its lower valence, for example, in the case of a titanium workpiece, a TiO film. This opinion appears to be substantiated by the fact that after titanium is processed in accordance with the invention, its surface color is dark grey, which is the color of TiO.

The invention will be further illustrated by presentation of the following specific examples:

EXAMPLE I Specimens of titanium were cleaned by immersing the same for 5 minutes in a bath consisting of 20 grams of chromic acid and 184 grams of sulfuric acid, the bath being at ambient temperature. From this bath the specimens, after rinsing, were transferred to an aqueous solution each liter of which contains grams of ferric chloride, 46 grams of sulfuric acid, 68 grams of nitric acid, and 25 grams of ammonium bifluoride, this second bath also being at ambient temperature and the specimens being immersed therein for 5 minutes. The specimens were rinsed with demineralized water after their removal from the second bath, and then were immersed for 10 minutes in a bath consisting of 10% by weight of sodium hydroxide, 4% by weight of sodium gluconate, 2% by weight of 1, 2, 3 trihydroxybenzene, and 84% by weight of water, the temperature of the bath being maintained at F. After being removed from this processing solution the specimens were again rinsed with demineralized water and dried by exposure to ambient air.

A number of the treated specimens were bonded together with Metlbond 329 (trademark of Whittaker Corporation) in overlapped end-to-end relation and tested for shear strength, which was found to average about 3600 p.s.1.

Other of the treated specimens were coated with epoxy polyamide paint. After this coating had dried, the specimens were lambda-scored and tape, sold by Minnesota Mining and Manufacturing Company as their tape No. 250, was applied over the scored areas and then pulled off without tearing the coating from the treated substrate, thus showing that there was a srong bond between the two materials.

EXAMPLE II Panels of titanium were processed by the same steps employed in the foregoing example, except that they were not dried after being removed from the alkaline processing bath and rinsed but instead were immersed in an electrolytic bath to thereby electroplate the same with copper and nickel. The electrodeposited layers on the substrate did not crack when the metal was bent with a radius of curvature of approximately A3 inch. Furthermore, sections of the plated metal were burnished to a highly polished finish without lifting the plated metals from the titanium substrate, indicating that the dark grey film which was placed on the srips by the alkaline bath treatment provides an effective surface finish for the attachment of electrodeposited metals to titanium.

It was also found that titanium treated as indicated in this example could be anodized to place strongly adhered, colored finishes on the surfaces thereof by the method disclosed in US. patent application Ser. No. 732,032, filed by Earl W. Kendall on May 27, 1968, and assigned by him to Rohr Corporation, the assignee of the present application.

EXAMPLE III Panels processed by two acid baths described in Example I were rinsed with water after being immersed in the second bath and were then immersed for 10 minutes in a bath consisting of 10% by weight of sodium hydroxide, 4% by weight of sodium gluconate, 2% by weight of 6-methyl naphthol 2, and 84% by weight of water, the temperature of the bath being maintained at 175 F. After being removed from this processing solution the panels were again rinsed with water and then dried.

Shear tensile strengh of panels bonded by the aforementioned Metlbond 329 adhesive was found to average about 3550 p.s.i., and specimens coated with epoxy polyamide paint, lambda-scored, and tested with tape as described in Example I were found to provide a good surface for bonding of the paint to the titanium substrate.

The 6-methyl naphthol-2 employed in the final processing bath of this example may be replaced by the same amount of other monomethyl substituted naphthols, such as 4-methyl naphthol-1 or 7-methyl naphthol-1, without reducing the effectiveness of the bath to produce a surface film to which other materials will firmly adhere. As noted previously, other alkali hydroxides may be substituted for the sodium hydroxide used in the given examples, and various chelating agents may be used instead of sodium gluconate.

It should be emphasized that the procedures described in Examples I through III can also be used to prepare the surfaces of titanium alloys, zirconium, hafnium, thorium, and alloys of the latter three metals for operations such as electroplating, anodizing, adhesive bonding, painting and the like. The embodiments of the invention described in the examples are therefore to be considered as in all respects illustrative and not restrictive, and all changes which come within the meaning and range of equivalence of the following claims are intended to be embraced therein.

What is claimed as new and useful and desired to be secured by US. Letters Patent is:

1. A solution for treating articles formed of titanium, zirconium, hafnium or thorium and alloys of these metals so that coatings will effectively bond to the surfaces thereof, consisting of:

about 1.0 to 35 weight percent of an alkali metal hydroxide;

about 1.0 to 4.0 weight percent of a chelating agent;

about 1.0 to 9.0 weight percent of a compound selected from the group consisting of the three trihydroxybenzenes and the monomethyl substituted naphthols; and

about 52.0 to 97.0 weight percent of water.

2. A solution for treating articles formed of titanium, zirconium, hafnium or thorium and alloys of these metals so that coatings will effectively bond to the surfaces thereof, consisting of:

about 1.0 to 35.0 weight percent of an alkali metal hydroxide;

about 1.0 to 4.0 weight percent of a chelating agent;

about 1.0 to 9.0 weight percent of at least one of the three trihydroxybenzenes; and

about 52.0 to 97.0 weight percent of water.

3. The solution defined in claim 2 wherein said alkali metal hydroxide is sodium hydroxide.

4. The solution defined in claim 2 wherein said chelating agent is sodium gluconate.

5. The solution defined in claim 2 wherein said trihydroxybenzene is 1, 2, 3 trihydroxybenzene.

6. The solution defined in claim 2 wherein said trihydroxybenzene is l, 2, 4 trihydroxybenzene.

7. The solution defined in claim 2 wherein said trihydroxybenzene is l, 3, 5 trihydroxybenzene.

8. A solution for treating articles formed of titanium, zirconium, hafnium and thorium and alloys of these metals so that coatings will effectively bond to the surfaces thereof, consisting of:

about 1.0 to 35.0 weight percent of an alkali metal hydroxide;

about 1.0 to 4.0 weight percent of a chelating agent;

about .5 to 2.0 weight percent of at least one methyl substituted naphthol;

about 58.0 to 97.0 weight percent of water.

9. The solution in claim 8 wherein the said substituted naphthol is 4-methyl naphthol-1.

10. The solution in claim 8 wherein the said substituted naphthol is 7-methyl naphthol-l.

11. The solution in claim 8 wherein the said substituted naphthol is 6-methyl naphthol-2.

12. A method of treating articles formed of titanium, zirconium, hafnium or thorium and alloys of these metal so that coatings will effectively bond the surfaces thereof, which comprises contacting said articles with a solution consisting of:

about 1.0 to 35 weight percent of an alkali metal hydroxide;

about 1.0 to 4.0 weight percent of a chelating agent;

about 1.0 to 9.0 weight percent of a compound selected from the group consisting of the three trihydroxybenizenes and the monomethyl substituted naphthols; an

about 52.0 to 97 .0 weight percent of water.

13. A method of treating articles formed of titanium, zirconium, hafnium or thorium and alloys of these metals so that coatings will effectively bond to the surfaces thereof, which comprises contacting said articles with a so1ution consisting of:

about 1.0 to 35.0 weight percent of an alkali hydroxide;

about 1.0 to 4.0 weight percent of a chelating agent;

about 1.0 to 9.0 weight percent of at least one of the three trihydroxybenzenes; and

about 52.0 to 97.0 weight percent of water.

14. The method defined in claim 13 wherein said articles are immersed in said solution for from 5 to 15 minutes.

15. The method defined in claim 13 wherein said solution is maintained at about to 200 F.

16. A method of treating articles formed of titanium, zirconium, hafnium or thorium and alloys of these metals so that coatings will effectively bond to the surfaces thereof, which comprises contacting said articles with a solution consisting of:

about 1.0 to 35.0 weight percent of an alkali hydroxide;

about 1.0 to 5.0 weight percent of a chelating agent;

about 0.5 to 2.0 weight percent of at least one methyl substituted naphthol; about 58.0 to 97.5 weight percent of water. 17. The method defined in claim 16 wherein said articles are immersed in said solution for from 5 to 15 minutes.

18. The method defined in claim 16 wherein said solution is maintained at about 175 to 200 F.

19. A method of treating articles formed of titanium, zirconium, hafnium, or thorium and alloys of these metals so that coatings will effectively bond to the surfaces thereof, which comprises the steps of sequentially:

immersing said articles in a solution consisting of about 6.0 to 12.5 weight percent of chromic acid and about 87.5 to 94.0 weight percent of sulfuric acid:

immersing said articles in a solution consisting of about 16 to 20 weight percent of ferric chloride, about 1 to 3.5 weight percent of sulfuric acid, about 1.2 to 4.0 weight percent of nitric acid, about 2.0 to 2.5 weight percent of ammonium bifluoride, and about 70.0 to 79.8 weight percent of water; and

immersing said articles in a solution consisting of 1.0 to 35.0 weight percent of alkali hydroxide, about 1.0 to 4.0 weight percent of a chelating agent, about 1.0 to 9.0 weight percent of at least one of the trihydroxybenzenes, and about 52.0 to 97.0 weight percent of water.

20. A method of treating articles formed of titanium, zirconium, hafnium or thorium and alloys of these metals so that coatings will effectively bond to the surfaces thereof, which comprises the steps of sequentially:

immersing said articles in a solution consisting of about 6.0 to 12.5 weight percent of chromic acid and about 87.5 to 94.0 weight percent of sulfuric acid;

immersing said articles in a solution consisting of about 16 to 20 weight percent of ferric chloride, about 1.0 to 3.5 weight percent of sulfuric acid, about 1.2 to 4.0 weight percent of nitric acid, about 2.0 to 2.5 weight percent of ammonium bifiuoride, and about 70.0 to 79.8 weight percent of water; and

immersing said articles in a solution consisting of about 1.0 to 35.0 weight percent of an alkali hydroxide, about 1.0 to 5.0 weight percent of a chelating agent, about 0.5 to 2.0 weight percent of at least one methyl substituted naphthol, and about 58.0 to 97.5 weight percent of water.

References Cited UNITED STATES PATENTS RALPH S. KENDALL, Primary Examiner US. Cl. X.R. 

